This invention relates to light emitting devices, and more particularly, to light emitting devices for use in displays or the like that excite one or more phosphors to produce a light output.
Currently there are many types of light emitting devices that are on the market. One of the original light emitting devices is a cathode ray tube. Cathode ray tubes have been used for many years in television and computer monitor applications.
Another type of light emitting device is a liquid crystal display (LCD). Due in part to its lower energy consumption and the lack of the need for an ultra-high vacuum environment, the LCD is widely used in portable computers and other special purpose applications, such as watches, calculators, and instrument panels. A limitation of an LCD is that back or external lighting is typically required. Further, some display characteristics such as color, brightness, and contrast can be viewing angle dependent.
Another type of light emitting device is a plasma display (PD). A conventional PD device includes a gaseous plasma UV source and a phosphor screen. The gaseous plasma UV source is used to excite the phosphors in the phosphor screen, which then produce the visible light output. A limitation of a PD device is that typical gaseous plasma UV sources require a vacuum environment and high voltages.
Another type of light emitting device is a field emitter display (FED) device. Like a PD device, a FED device excites phosphors to produce a visible light output. However, unlike a PD device, the FED device uses a field induced electron emission. Field induced electron emission can typically only be accomplished in a vacuum environment. Thus, like a PD device, the FED device requires a vacuum environment.
Another type of light emitting device is an electroluminescence (EL) device. Like the PD and FED devices, an EL device excites phosphors to produce a visible light output. Unlike the PD and FED devices, however, the EL device excites the phosphor using direct injection of current. Direct injection of current does not require a vacuum environment. However, EL devices typically have a limited color emission spectrum.
An improved light emitting device is disclosed in WO 97/48138 to Mensz et al. Mensz et al. discloses a solid-state GaN-based UV Light Emitting Diode (LED) that provides a UV radiation to a phosphor layer. The UV LED excites the phosphor layer to produce a visible light output. Mensz et al. has a number of advantages over the LED, PD, FED and EL type devices. First, Mensz et al. do not require back or external lighting, as required by LCD type devices. Further, Mensz et al. does not require a vacuum and/or high voltage power source, as required by PD and FED type devices. Finally, the solid-state GaN-based UV LED with excited phosphors of Mensz et al. may provide a wider range of colors than an EL type device.
A limitation of Mensz et al. is that a significant amount of UV radiation may not be converted to visible light. In some embodiments, a substantial amount of UV radiation is not directed at the phosphor layer. This may reduce the efficiency of the device. Also, some of the UV radiation that actually enters the phosphor passes through the phosphor and out of the other side, without exciting the phosphor. This pass-through UV radiation also reduces the efficiency of the device. Further, in an array of devices, the UV radiation escaping from one device can impinge on the phosphors of neighboring devices, causing unwanted emission of visible light from the neighboring devices. This optical cross talk is undesirable in many display applications.
The present invention overcomes many of the disadvantages of the prior art by providing a more efficient solid-state light emitting device that excites phosphors to produce a visible light output. To accomplish this, the present invention contemplates providing a reflector adjacent to the phosphor layer for reflecting at least some of the UV radiation that passes through the phosphor, back into the phosphor. The reflector may also reflect at least some of the visible light that is emitted by the phosphor toward a designated light output. In another embodiment, the radiation source is at least partially surrounded by the visible light emitting phosphor. This configuration may cause more of the radiation that is emitted from the active region to reach and interact with the phosphor material.
In one illustrative embodiment of the present invention, a visible light-emitting phosphor layer is positioned between a radiation source and a reflector. The radiation source is preferably a UV light emitting diode (LED), which is formed on a transparent substrate. The reflector is positioned above the phosphor layer, and reflects at least some of the radiation that passes through the phosphor, back into the phosphor. The reflector may also reflect at least some of the visible light that is emitted from the phosphor toward the transparent substrate for viewing. In some embodiments, the reflector may also be conductive and serve as both a reflector and an interconnect layer to the radiation source.
In a second illustrative embodiment of the present invention, the radiation source has an active region that is at least partially surrounded by a visible light emitting phosphor. When properly biased, the active region provides an excitation radiation to the visible light emitting phosphor. Because the active region is at least partially surrounded by the visible light emitting phosphor, more of the radiation that is emitted by the active region is allowed to interact with, and thus excite, the phosphor material. This helps increase the overall efficiency of the light emitting device.
To surround the active region with a visible light emitting phosphor, the radiation source may have a column-shaped portion with a top surface and one or more side walls. The active region is located somewhere in the column-shaped portion. The phosphor layer is then provided around the one or more side walls of the column-shaped portion, thereby surrounding at least part of the circumference of the active region. A reflector may be provided over and around the phosphor layer to help improve the efficiency of the device.
It is contemplated that the above visible light emitting devices may be provided in an array configuration to form a display. In such a display, an array of phosphor segments, each including one or more excitable, visible light-emitting phosphors, may be positioned adjacent to a corresponding array of radiation sources. The radiation sources preferably can be individually addressed via a number of row and column contact layers. Accordingly, the radiation sources can be controlled to provide radiation to only selected phosphor segments. The row or column contact layer may also serve as a reflector adjacent to each of the phosphor segments. In the embodiment where the radiation source has a column-shaped portion with a top surface and one or more side walls, it is contemplated that the reflector may extend over and around the phosphor layer. This may reduce the UV radiation produced in one pixel from impinging on the phosphors of a neighboring pixel, which translates into reduced optical cross talk between pixels.